SEOUL (Reuters) - South Korea's three opposition parties introduced a bill on Saturday for parliament to impeach President Park Geun-hye who, tainted by a corruption scandal, could become the first democratically elected leader to leave office early in disgrace.

The bill, signed by 171 members of the 300-seat unicameral assembly, said Park violated the constitution and criminal law by abusing her presidential power.

"We hereby propose impeachment proceedings to protect the constitution and restore constitutional order by removing President Park Geun-hye from office," it said.

"The will of the people that President Park Geun-hye should be made to cease exercising the duty of president is clear. The sovereign will has been shown clearly through rallies and protests peacefully attended by countless numbers of the public regardless of generation, ideology or background."

Park has come under intense pressure to step down, with hundreds of thousands of people streaming into the streets demanding her resignation at successive weekend rallies, which have remained peaceful.

Another large protest rally demanding she step down immediately is planned for later Saturday, the sixth weekend protest in succession.

Park is accused of colluding with a friend, Choi Soon-sil, who has been accused of abuse of power, to put undue pressure on conglomerates to contribute money to foundations that were set up to promote her policy initiatives.

Park has denied wrongdoing but has apologized to the nation.

The three opposition parties said on Friday they will hold a parliamentary impeachment vote on Park on Dec. 9. A bill once introduced is reviewed by the parliament secretariat, a formality, before it can be reported to the plenary session.

The bill was expected to go to the session on Thursday, according to the parties.

The three opposition parties have enough members to bring the impeachment but will need 28 members from Park's Saenuri Party to bring the vote to the two-thirds majority required to pass the bill.

It was not clear whether enough Saenuri members would support the vote. There are seven non-party affiliated members plus the speaker who are expected to give their backing.

Some Saenuri members had earlier said they would join the opposition parties to impeach Park but changed their position after Park offered to quit, saying she should be given the chance to step down of her own accord by April.